An optical fiber conventionally includes an optical core, which transmits and/or amplifies an optical signal, and an optical cladding, which confines the optical signal within the core. Accordingly, the refractive index of the core nc is typically greater than the refractive index of the outer cladding ng (i.e., nc>ng).
The refractive index profile is generally classified according to the graphical appearance of the function that associates the refractive index with the radius of the optical fiber. Conventionally, the distance r to the center of the optical fiber is shown on the x-axis, and the difference between the refractive index (at radius r) and the refractive index of the optical fiber's outer cladding is shown on the y-axis. The refractive index profile is referred to as a “step” profile, a “trapezoidal” profile, an “alpha” profile, or a “triangular” profile for graphs having the respective shapes of a step, a trapezoid, an alpha, or a triangle. These curves are generally representative of the optical fiber's theoretical profile (i.e., the set profile). Constraints in the manufacture of the optical fiber, however, may result in a slightly different actual profile.
Generally speaking, two main categories of optical fibers exist: multimode fibers and single-mode fibers. In a multimode fiber, for a given wavelength, several optical modes are propagated simultaneously along the optical fiber, whereas in a single-mode fiber the higher order modes are strongly attenuated. The typical diameter of a single-mode or multimode optical fiber is 125 microns. The core of a multimode fiber typically has a diameter of between about 50 microns and 62.5 microns, whereas the core of a single-mode fiber typically has a diameter of between about 6 microns and 9 microns. Multimode systems are generally less expensive than single-mode systems because multimode light sources, connectors, and maintenance can be obtained at a lower cost.
In a multimode fiber, the difference between the propagation times, or group delay times, of the several modes along the optical fiber determine the bandwidth of the optical fiber. In particular, for the same propagation medium (i.e., in a step-index-type multimode fiber), the different modes have different group delay times. This difference in group delay times results in a time lag (i.e., a delay) between the pulses propagating along different radial offsets of the optical fiber. This delay causes a broadening of the resulting light pulse. Broadening of the light pulse (i) increases the risk of the pulse being superimposed onto a following pulse and (ii) reduces the bandwidth (i.e., data rate) supported by the optical fiber. The bandwidth, therefore, is directly linked to the group delay time of the optical modes propagating in the multimode core of the optical fiber. Thus, to guarantee a broad bandwidth, it is desirable for the group delay times of all the modes to be identical. Stated differently, the intermodal dispersion should be minimized, or even eliminated, for a given wavelength.
To reduce intermodal dispersion, the multimode optical fibers used in telecommunications generally have a core with a refractive index that decreases progressively from the center of the optical fiber to its interface with a cladding (i.e., an “alpha” core profile). Such an optical fiber has been used for a number of years, and its characteristics have been described in “Multimode Theory of Graded-Core Fibers” by D. Gloge et al., Bell system Technical Journal 1973, pp. 1563-1578, and summarized in “Comprehensive Theory of Dispersion in Graded-Index Optical Fibers” by G. Yabre, Journal of Lightwave Technology, February 2000, Vol. 18, No. 2, pp. 166-177. Each of the above-referenced articles is hereby incorporated by reference in its entirety.
An optical fiber having a graded-index profile (i.e., an alpha-index profile) typically has a graded-index core surrounded by a cladding. The alpha-index profile follows a power law for parameter α, which can be described by a relationship between the refractive index value n and the distance r from the center of the optical fiber according to the following equation:
      n    ⁡          (      r      )        =            n      max        ⁢                  1        -                  2          ⁢                                          ⁢                                    Δ              ⁡                              (                                  r                                      r                    1                                                  )                                      α                              
wherein,
α≧1, and α is a non-dimensional parameter that is indicative of the shape of the refractive index profile;
nmax is the maximum refractive index of the multimode optical fiber's core;
r1 is the radius of the multimode optical fiber's core; and
  Δ  =            (                        n          max          2                -                  n          min          2                    )              2      ⁢              n        max        2            
where nmin is the minimum refractive index of the multimode core.
A multimode fiber with a graded index (i.e., an alpha profile) therefore has a core profile with a rotational symmetry such that along any radial direction of the optical fiber the value of the refractive index decreases continuously from the center of the optical fiber to its periphery. When a multimode light signal propagates in such a graded-index core, the different optical modes experience differing propagation mediums (i.e., because of the varying refractive indices), which affects the propagation speed of each optical mode differently. Thus, by adjusting the value of the parameter α, it is possible to obtain a group delay time that is virtually equal for all of the modes. Stated differently, the refractive index profile can be modified to reduce or even eliminate intermodal dispersion.
As used herein, a “standard graded-index optical fiber” is an optical fiber with an alpha-profile satisfying the ITU-T G.651.1 recommendations and the OM3 standard. Furthermore, a “standard graded-index optical fiber” has a central core with a minimum refractive index approximately equal to the refractive index of the optical fiber's outer optical cladding. For example, the refractive index difference between the central core's minimum refractive index and the outer optical cladding's refractive index is less than 1×10−3 (or even zero).
Multimode fibers have been the subject of international standardization under the ITU-T G.651.1 recommendations, which, in particular, define criteria (e.g., bandwidth, numerical aperture, and core diameter) that relate to the requirements for optical fiber compatibility. The ITU-T G.651.1 recommendations are hereby incorporated by reference in their entirety.
In addition, the OM3 standard has been adopted to meet the demands of high-bandwidth applications (i.e., a data rate higher than 1 GbE) over long distances (i.e., distances greater than 300 m). The OM3 standard is hereby incorporated by reference in its entirety. With the development of high-bandwidth applications, the average core diameter for multimode fibers has been reduced from 62.5 microns to 50 microns.
Multimode fibers are commonly used for short-distance applications requiring a broad bandwidth, such as local area networks (LANs), in which the optical fibers may be subjected to accidental or otherwise unintended bending. Bending, however, can modify the mode power distribution and the bandwidth of the fiber.
It is therefore desirable to design multimode fibers that (i) are unaffected by bends having a radius of curvature of less than 10 millimeters, (ii) are compatible with standard graded-index fibers, and (iii) allow high-speed transmission of 10 Gb/s in 10 GbE (Gigabit Ethernet) systems.
One proposed solution to such a problem involves adjusting the light-injection conditions from a light source. Because all of the modes are affected differently by bending, it would be sufficient to modify the light-injection conditions such that the only modes coupled with the injected light are modes unaffected by bends. In practice, however, the light source and the coupling device between the light source and the optical fiber require launch conditions that prevent the adjustment of the injection conditions.
Moreover, restricting the coupling to the lowest order modes reduces the power coupling efficiency. Such a phenomenon can be accompanied by an increase in the impact of the mode partition noise on the light transmitted in the fiber. The increase appears when laser sources are used in combination with the optical fiber and, in particular, if VCSEL (Vertical Cavity Surface Emitting Laser) diodes allowing a transmission of 10 GbE are used. The mode partition noise corresponds to “jitter” of the signal phase due to the combined effects of changing the main mode of the optical source (i.e., “mode hopping”) and intramodal distortions in the fiber. The change of the main mode is a sudden jump in the optical frequency of the optical source, associated with transitions between the different modes of the resonator. Thus, changing the main mode of the optical source leads to random modifications in the wavelength which affects the group velocity and therefore the propagation time. Over the length of the fiber, the cumulative effect of this variation of the group velocity is an induced phase jitter (i.e., mode partition noise).
Another known solution proposes applying additional bends to the multimode fiber. For example, European Patent No. 1,727,302 and its counterpart U.S. Patent Publication No. 2009/010596 A1, each of which is hereby incorporated by reference in its entirety, disclose an access network that includes a bent multimode fiber. Bending a multimode fiber in this manner reduces the transmission loss due to a bend in the remaining network. Applying additional bends to a multimode fiber, however, can also reduce power coupling efficiency with an increase in the impact of the mode partition noise if VCSELS allowing a 10-GbE transmission are used.
Another solution is a dedicated fiber architecture (i.e., a specific optical index profile). For example, adding a depressed trench between the core and the cladding can reduce the bending losses of a graded-index multimode fiber. International Publication No. 2008/085851 and its counterpart U.S. Patent Publication No. 2008/0166094 A1, each of which is hereby incorporated by reference in its entirety, describe a graded-index core surrounded by a depressed trench.
Adding a depressed trench between the core and the cladding, however, poses several manufacturing problems. Typically, the central core (i.e., the “alpha” core), the cladding, and at least a portion of the external protective cladding are obtained by chemical vapor deposition (CVD) in a silica tube. The cladding or external protective cladding is constituted by the tube and an overcladding of the tube (e.g., an overcladding of natural or doped silica). The overcladding may be obtained by any other deposition technique (e.g., VAD or OVD). However, the manufacturing methods disclosed in International Publication No. 2008/085851 and U.S. Patent Publication No. 2008/0166094 A1 require a broad deposition zone, which increases the cost of chemical vapor deposition techniques. In fact, this leads to the production of a smaller length of fiber per core rod.
Moreover, the addition of a depressed trench results in the appearance of supplementary propagation modes known as leaky modes. The leaky modes have effective refractive indices that are lower than those of the guided modes. These leaky modes increase the numerical aperture of graded-index optical fibers having a depressed trench in comparison to the graded-index optical fibers without a depressed trench. A difference in numerical aperture can cause losses during connections within a system that employs both (i) depressed trench graded-index fibers and (ii) graded-index fibers without a depressed trench.
Therefore, a need exists for a graded-index optical fiber having reduced bending losses and reduced coupling losses when connected to a standard graded-index fiber.